A variety of high performance thermoplastic polymers are commercially produced from monomers of a non-biobased, non-renewable nature. Commercially desirable thermoplastic polymers typically have a high glass transition temperature (Tg) and a high molecular weight (MW), and preferably are semi-crystalline in character so that the materials will have sufficient strength when used at high temperatures. Desirable attributes further include high thermal stability and low color. Petrochemical examples of commercially valuable thermoplastics include polyethylene terephthalate (PET, with a Tg of 80° C., and a Tm of 260° C.), polybutylene terephthalate (PBT, having a Tg of 40° C., and a Tm of 220° C.) and bisphenol-A polycarbonate (PC, with a Tg of 150° C.).
Functionally equivalent biobased polymeric materials have been increasingly sought in recent years as the manufacturing cost of conventional petroleum-based polymers has increased. Over the years various classes of rigid biobased, difunctional monomers have been developed from which high performance, biobased thermoplastic polyesters could be made, including 2,5-furandicarboxylic acid (FDCA), acetalized aldaric acid and alditols, and isohexides.
Of these materials, the isohexides are bicyclic, rigid diols that differ only in the orientation of the hydroxyl groups at C2 and C5. They can be obtained by cyclodehydration of the respective hexitols, i.e. isomannide (endo-endo) from mannitol, isosorbide (exo-endo) from sorbitol and isoidide (exo-exo) from iditol.
Mainly due to the limited availability of isomannide and isoidide, most of the scientific and patent literature on isohexide polymers describes the effects of incorporation of isosorbide (which is commercially produced on a small scale). This literature has established that incorporating isosorbide in polyesters in general produces a significant increase in the Tg of the ensuing polymers, which could widen the scope of application of these materials.
However, thus far several drawbacks have hampered the successful commercialization of isohexide based polymers. The secondary hydroxyl groups are less reactive than primary groups, resulting in lower reactivity, and hence require harsh (though industrially common) melt polymerization conditions in order to build up molecular weight. However, such conditions also lead to increased degradation and color formation. Furthermore, the presence of two hydroxyl groups with a different spatial orientation as in isosorbide leads to the formation of random, stereo-irregular polymers, which prohibits crystallization.
Isoidide on the other hand has a symmetrical arrangement of the two hydroxyl groups, and efforts have been made previously to prepare polymers based on isoidide, though as noted previously these efforts have been limited in extent because of isoidide's limited availability.
Thiem and Lüders (Thiem et al., Polym. Bull., vol. 11, pp. 365-369 (Berlin, 1984); Thiem et al., Starch/Staerke, vol. 36, pp. 170-176 (1984)) were the first to report on the synthesis of polyisoidide terephthalate (PIIT) by melt polymerization of the diol with terephthaloyl chloride (TDC) at 180° C. The resulting polymer had a number average molecular weight Mn of 3,800 (by membrane osmometry), a Tg of 153° C. and a Tm of 192° C.
Later, Storbeck et al. (Storbeck et al., Makromol. Chem., col. 194, pp. 53-64 (1993)) prepared a semi-crystalline PIIT by solution polymerization from the diol and TDC (toluene, pyridine, 100° C.), reporting however a polymer with a significantly higher number average molecular weight Mn of 14,500 (by membrane osmometry), a Tg of 209° C., and a Tm of 261° C.
Of greater relevance to the materials of the present invention, Storbeck and Ballauf also reported the synthesis and characterization of a polyester of isoidide and FDCA (Storbeck et al., Polymer, vol. 34, pp. 5003-5006 (1993)). This furanoate polyester was obtained by solution polymerization of the diol with the acid chloride of FDCA (tetrachloroethane, pyridine, 25° C.) with a Mn of 21,500 (by membrane osmometry) and a Tg of 196° C. Although wide angle x-ray scattering (WAXS) analysis suggested a degree (very low) of crystallinity, no Tm was reported.
More recently, moreover, Gomes et al. (Gomes et al., J. Polym. Sci., Part A: Polym. Chem., vol. 49, pp. 3759-3768 (2011)) reported the preparation of the same isoidide furanoate polyester by a slightly adapted procedure, and mentions nothing of any observed crystallinity; the isoidide furanoate polyester made by Gomes et al. had a number average molecular weight Mn of 5,650 and a Tg of 140° C.
Consequently, while the literature to date does demonstrate that polyisoidide terephthalate polyesters have been made which demonstrate the desired semi-crystalline nature, there appears to be no precedent for a fully biobased semi-crystalline polyisoiside furanoate by melt polymerization, using FDCA in place of purified terephthalic acid (PTA).